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Abstract:

The present invention relates to the field of laser machining and provides
an optical lens, comprising a lens group and a diaphragm. The diaphragm
is located in front of the lens group. The lens group comprises three
lenses, respectively the first, the second and the third lens, which are
sequentially arranged as a "negative-positive-positive" separated focal
power system, wherein the first lens is a negative meniscus lens, the
second lens is a positive meniscus lens, the third lens is a positive
meniscus lens, all the curved surfaces of the first lens, the second lens
and the third lens are bent in the direction towards the diaphragm, and
the focal length of the entire optical system is f, the focal lengths of
the first, the second and the third lens are respectively f1, f2, and f3,
and the ratio between the focal length of each lens and the focal length
f of the entire optical system satisfies the following requirement:
-0.7<f1/f<-0.4; 0.9<f2/f<1.1; 0.7<f3/f<0.9. The optical
lens provided in the present invention adopts a lens group consisting of
a negative meniscus lens, a positive meniscus lens, and a positive
meniscus lens arranged in sequence, so as to make the spherical
aberration, the image aberration and the curvature of field of the system
all achieve a better balance, to make the result of imaging well and to
make the imaging uniform all over the image plane. Moreover, the
structure upon being applied to the fθ ens with a large aperture
incidence achieves a comparatively good result in respect of
miniaturization, and the incident light aperture at the system is
relatively large.

Claims:

1. An optical lens, comprising a lens group and a diaphragm, wherein: said
diaphragm is located in front of said lens group, said lens group
comprising three lenses, respectively the first, the second and the third
lens, which are sequentially arranged as a "negative-positive-positive"
separated focal power system, wherein said first lens is a negative
meniscus lens, said second lens is a positive meniscus lens, and said
third lens is a positive meniscus lens, all the curved surfaces of said
first lens, said second lens and said third lens are bent in the
direction towards said diaphragm, and the focal length of the entire
optical system is f, the focal lengths of the first, the second and the
third lens are respectively f1, f2, and f3, and the ratio between the
focal length of each lens and the focal length f of the entire optical
system satisfies the following
requirement:-0.7<f1/f<-0.40.9<f2/f<1.10.7<f3/f<0.9

2. The optical lens according to claim 1, wherein: f1/f=-0.57, f2/f=0.95,
and f3/f=0.79.

3. The optical lens according to claim 1, wherein: the distance between
said first lens and said diaphragm is 25-60 mm.

Description:

[0001]This application is being filed as a PCT International Patent
Application in the name of HAN'S LASER TECHNOLOGY CO., LTD and claims the
benefit of Chinese Patent Application No. 200810066902.X, filed Apr. 28,
2008, which is herein incorporated by reference in the entirety.

TECHNICAL FIELD

[0002]The present invention relates to the field of laser machining, in
particular, to an optical lens.

BACKGROUND

[0003]Nowadays, application of laser is deeply engaged in every aspect of
the modern life of human beings. In the application of laser, various
applied optical systems required for meeting the demands of various
processes are indispensable. Presently, the laser marking machine
existing in the market are gradually replacing various ink writers,
screen printers and the like due to the characteristics of its fast
speed, great flexibility, needing no consumables, permanent marking and
so on.

[0004]The F-theta (fθ) lens is a kind of photographic objective lens
that has a large field of view, medium and small-sized aperture, and
medium and long-sized focal length. Choosing a photographic objective
lens of a "three-piece" type will be relatively suitable concerning the
parameters it is to possess. The flat-field optical lens for laser
scanning is called fθ lens, and this lens achieves that, when the
laser beam is scanning at a constant angular velocity, focusing point of
the light beam transmitting through this lens on the image plane is also
moving at a uniform speed, which determines that scanning angle of the
light beam and image height of the focusing point on the image plane will
establish a linear relationship. A galvanometer laser marking machine is
realized due to the present of the fθ ens.

[0005]FIG. 1 is a typical fθ lens optical system in the conventional
art, in which the light beam is reflected by a reflector that is scanning
at an angular velocity rotating at uniform speed, then focused on the
image plane by the fθ lens, that is, the light beam sequentially
passes two galvanometers 1 and 2 that are respectively rotating about the
x-axis and the y-axis, and at last passes through the fθ lens 3 and
is focused on the image plane 4, so as to form an image by scanning with
the galvanometer. The fθ lens 3 is a flat-field focusing lens. It
is required that image height η on the image plane has a linear
relationship with the scanning angle θ of the X galvanometer 1 and
the Y galvanometer 2 upon marking, that is, η=f*θ(Sr), wherein,
assuming that incident angle of light with respect to the fθ lens
at a certain point of time is θ, the image height of the generated
image with respect to the center point is η, then there must exist a
linear relationship therebetween, that is, η=k*f*θ. Wherein, k
is a constant; f is the focal length of the fθ lens, which is a
fixed value for a particular lens; and θ is the scanning angle of
the galvanometer (in radians).

[0006]According to the theory of Gauss optical imaging, it is known that
the image height η has a following relationship with the focal length
f of the lens and the angle of rotation θ of the light beam:
η=f*tgθ. However, normally, there is always a certain
distortion in an imaging system. Assuming that in the correction of
aberration of the optical design, a distortion Δη is
intentionally introduced in, so as to satisfy the relationship as shown
in the formula below: η=f*tgθ-66 η=k*f*θ, then the
requirement that the object-image relationship of the fθ lens is a
linear relationship may be achieved. Therefore, it is derived that
Δη=f*tgθ-k*f*θ=f(tgθ-k*θ), wherein
Δη is a positive value, and the fθ lens is an optical
system with a negative distortion. Therefore, it is required that the
system has a relatively large negative distortion when the angle is
relatively large.

[0007]Meanwhile, the diaphragm of the fθ lens which is outside of
the lens is a typical non-symmetrical optical system. When issue of the
balance of the vertical aberration is concerned in the design of the
existing products, generally, the design is carried out by adopting the
symmetrical structure of Pitzval, so as to achieve the correction of the
vertical aberration. However, on the contrary, it is very difficult to
correct the vertical aberration very well by adopting the symmetrical
structure of Pitzval to design this kind of non-symmetrical optical
system.

[0008]In addition, another feature of the fθ lens is to require that
all the focus points within the range of imaging must have a similar
quality of focusing and no vignetting is allowed, so as to assure that
all the image points are consistent. In order to increase the service
life of the lens, it is required to adopt no cemented lens when it is
used in a laser applied light path due to very large energy density of
the laser sometimes.

SUMMARY

Technical Problem

[0009]An aspect of the present invention is to provide an optical lens,
which has uniform imaging in all the field of view with no vignetting and
is used for laser.

Technical Solution

[0010]The technical solution adopted in the present invention is to
provide an optical lens, which comprises a lens group and a diaphragm.
The diaphragm is located in front of the lens group. The lens group
comprises three lenses, respectively the first, the second and the third
lens which are sequentially arranged as a "negative-positive-positive"
separated focal power system, wherein, the first lens is a negative
meniscus lens, the second lens is a positive meniscus lens, the third
lens is a positive meniscus lens. All the curved surfaces of the first
lens, the second lens and the third lens are bent in the direction
towards the diaphragm, and the focal length of the entire optical system
is f, the focal lengths of the first, the second and the third lens are
respectively f1, f2, and f3, and the ratio between the focal length of
each lens and the focal length f of the entire optical system satisfies
the following requirement:

-0.7<f1/f<-0.4

0.9<f2/f<1.1

0.7<f3/f<0.9

ADVANTAGEOUS EFFECT

[0011]The optical lens provided in an present invention adopts a lens
group comprising a negative meniscus lens, a positive meniscus lens, and
a positive meniscus lens arranged in sequence, so as to make the
spherical aberration, the image aberration and the curvature of field of
the system all achieve a better balance, to make a good result of imaging
and to make the imaging uniform all over the image plane. Moreover, the
structure upon being applied to the fθ lens with a large aperture
incidence achieves a comparatively good result in respect of
miniaturization. In addition, the incident beam aperture is relatively
large in the system, and the structure in a form is different from the
general fθ optical system, so as to make the product of this system
capable of being exchanged with a general product having a small incident
beam aperture, thereby achieving a miniaturized design.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic view of a typical fθ lens optical system
provided in the conventional art;

[0013]FIG. 2 is a schematic and structural view of an optical lens
provided in an embodiment of the present invention;

[0014]FIG. 3 is a light trace diagram of an optical lens provided in a
first embodiment of the present invention;

[0015]FIG. 4 is a distribution diagram of astigmatism, curvature of field
and distortion of the optical lens provided in the first embodiment of
the present invention;

[0016]FIG. 5 is a curve distribution diagram of linearity difference of
the optical lens provided in the first embodiment of the present
invention;

[0017]FIG. 6 is a curve distribution diagram of ligh path difference of
the optical lens provided in the first embodiment of the present
invention when the field of view thereof is respectively 0, 0.3, 0.5,
0.7, 0.85 and 0.1;

[0018]FIG. 7 is an MTF distribution diagram of optical transfer function
of the optical lens provided by the first embodiment of the present
invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019]The present invention is further explained in combination with the
accompanying drawings and the specific embodiments thereof as follows.

[0020]The fθ lens is a kind of photographic objective lens that has
a large field of view, medium and small-sized aperture, and medium and
long-sized focal length. Choosing a photographic objective lens with the
"three-piece" type will be relatively suitable concerning the parameters
it is to possess. A "negative-positive-positive" focal power distribution
pattern is adopted herein, and the distortion generated by the entrance
pupil thereof outside of the lens is needed by the fθ lens. The
distortion may easily meet the requirement of the fθ lens and is a
kind of "non-deformed" marking. Meanwhile, it is a photographic objective
lens with a large field of view. It is an objective lens that has a flat
image field similar to the photographic objective lens.

[0021]As shown in FIG. 2, the technical solution adopted in the present
invention is to provide an optical fθlens for laser machining,
which comprises a lens group and a diaphragm (galvanometer) 1 which is
located in front of the lens group. The lens group comprises three
lenses, respectively the first, the second and the third lens L1, L2 and
L3, and a three-piece "negative-positive-positive" focal power
distribution is adopted for the design, wherein the material used for
each of the three lenses is colloid, through which the relevant
aberrations are corrected by a lens with double cemented surfaces in a
high-power laser system.

[0022]Wherein the focal power of the first lens 1/f1 is negative, and the
focal power of the second lens 1/f2 and that of the third lens 1/f3 are
both positive, wherein the ratio between the focal length of each lens
and the focal length f of the entire optical system satisfies the
following requirement:

-0.7<f1/f<-0.4

0.9<f2/f<1.1

0.7<f3/f<0.9

wherein, the focal length of the entire optical system is f, and the focal
lengths of the first, the second and the third lens are respectively f1,
f2, and f3.

[0023]Wherein, the distance d0 of the first lens L1 from the diaphragm
(galvanometer) 1 is 25-60 mm, the first lens L1 is a negative meniscus
lens, the second lens L2 is a positive meniscus lens, the third lens L3
is a positive meniscus lens. All the curved surfaces of the first lens
L1, the second lens L2 and the third lens L3 are bent in the direction
towards the diaphragm (galvanometer) 1; and the distance of the third
lens L3 from the focal plane 4 is d6.

[0024]Sometimes an optical window composed of parallel flat boards is
added at any position in the light exiting direction of the lens group
for protecting the outwardly exposed lens or for any other purpose when
the above lens group is put into practice. The addition of the optical
window under the above parameter conditions is covered within the concept
of the present invention.

[0025]The beneficial effect achieved by adopting the above design is as
follows. The relationship adopted for assigning the focal lengths of this
system makes the radial size of the lens very small, facilitating the
standardization of the exterior shape and the mounting size in a system
with a large diameter of incident light. A relatively large distortion is
achieved by means of a non-symmetrical structure, and the limitation that
the distortion is relatively small in a symmetrical structure is
overcome, such that the design may very easily satisfy the requirement
that object-image relationship is linear. A separated lens system is
adopted without any cemented interface, thus the influences caused by
aging of the glue or damage by the laser when used in a strong laser
applied optical path are avoided, so the stability and service life of
the lens are improved. Meanwhile, various aberrations affecting the
imaging quality are relatively well corrected simply by adopting a
three-piece separated lens, thus the cost of the lens is greatly reduced.

[0026]The specific structure and parameters thereof are expressed as
follows:

The system is composed of three lenses L1, L2 and L3, the lens L1 is
composed of two curved surfaces S1 and S2 which are respectively with a
curvature radius of R1 and R2, the center thickness thereof is d1, and
the optical parameter of the material is Nd1:Vd1; the lens L2 is composed
of two curved surfaces S3 and S4 which are respectively with a curvature
radius of R3 and R4, the center thickness thereof is d3, and the optical
parameter of the material is Nd3:Vd3; the lens L3 is composed of two
curved surfaces S5 and S6 which are respectively with a curvature radius
of R5 and R6, the center thickness thereof is d5, and the optical
parameter of the material is Nd5:Vd5; the interval between the first lens
L1 and the second lens L2 is d2, and the interval between the second lens
L2 and the third lens L3 is d4.

[0027]With the above parameters, a group of lens is devised according to
the present invention, the specific parameters thereof are respectively
as shown below:

[0028]The first lens L1 is composed of two curved surfaces S1 and S2 which
are respectively with a curvature radius of R1=-54.455 mm and R2=-205.1
mm, the center thickness d1 thereof on the optical axis is equal to 9 mm,
and the optical parameters of the material Nd1:Vd1 is about 1.52/64; the
second lens L2 is composed of two curved surfaces S3 and S4 which are
respectively with a curvature radius of R3=-133.02 mm and R4=-81.693 mm,
the center thickness d3 thereof on the optical axis is equal to 13.8 mm,
and the optical parameter of the material Nd3:Vd3 is about 1.8/25.4; the
third lens L3 is composed of two curved surfaces S5 and S6 which are
respectively with the curvature radius of R5=-899.934 mm and R6=-135.486
mm, the center thickness d5 thereof on the optical axis is equal to 8 mm,
and the optical parameter of the material Nd5:Vd5 is about 1.8/25.4; the
interval d2 between the first lens L1 and the second lens L2 on the
optical axis is equal to 2.5 mm, the interval d4 between the second lens
L2 and the third lens L3 on the optical axis is equal to 0.5 mm, and the
distance d6 of the third lens L3 from the image plane on the optical axis
is equal to 308.5 mm. The above parameters are listed as follows:

[0031]FIG. 3 is a light trace diagram of a first embodiment, illustrating
the layout of the lens of the product of this embodiment. FIG. 4 is a
distribution diagram of the astigmatism, the curvature of field and the
distortion (A is the distribution diagram of the astigmatism and the
curvature of field, B is the distribution diagram of the distribution),
it is seen that the astigmatism and the curvature of field of the system
of this embodiment are very well corrected, as shown in the figure. FIG.
5 is a curve distribution diagram of the linearity difference, the linear
error of the system is at maximum within ±0.5%, thus the equation of
the object-image relationship of the F-theta lens is relatively well
realized. FIG. 6 is a light path difference diagram when the field of
view is respectively 0, 0.3, 0.5, 0.7, 0.85 and 0.1, the light path
difference is at maximum not larger than ±0.2λ, showing that the
aberration of the system of this embodiment is relatively well corrected.
FIG. 7 is an MTF distribution diagram of the optical transfer function,
from which it is seen that the MTF value of each field of view is
relatively consistent, showing that the imaging over all field of view is
uniform.

[0032]It is shown from each of the above figures that: the astigmatism and
the curvature of field of the system are very well corrected, the light
path difference is at maximum not larger than ±0.2λ, and
according to the MTF distribution diagram of the optical transfer
function, the MTF value of each field of view is relatively consistent,
showing that the imaging over all field of view is uniform with no
vignetting, moreover, the incident beam aperture is relatively large in
the system, and a structure in a form different from the general fθ
optical system is adopted, so as to make the product of this system
capable of being exchanged with a general product having a small incident
beam aperture, thereby achieving a miniaturized design. Further, the
relevant aberrations are corrected by adopting a lens with two cemented
surfaces in a high-power laser system.